I was frustrated that I can't preview instruments using computer keys, but I found the workaround (see answer)
1) For clips, simply press Right Arrow once the clip is selected, so Down Arrow then Right Arrow will preview .alc files
2) This is simply a matter of double-clicking an instrument in the sidebar, as if you were going to use it. When the instrument loads in, it's highlighted, so you can easily swap it out by double-clicking another browser instrument!
Not as fast as pressing up/down arrows in the browser, but synths are more complicated than wavs
You can even precede the instrument with a C Minor scale (or transpose to A for example), and as long as you have the instrument highlighted, a double click in the browser will replace it, allowing you to experiment in key.